Although glucosinolate hydrolysis products may alter the metabolism or activity of sex hormones in ways that could inhibit the development of hormone-sensitive cancers, evidence of an inverse association between cruciferous vegetable intake and breast or prostate cancer in humans is limited and inconsistent. (More information)

Many organizations, including the National Cancer Institute, recommend eating a variety of fruit and vegetables daily; the recommended serving number depends on age, sex, and physical activity level. However, separate recommendations for cruciferous vegetables have not been established. (More information)

Introduction

Cruciferous or Brassica vegetables are so named because they come from plants in the family known to botanists as Cruciferae or alternately, Brassicaceae. Many commonly consumed cruciferous vegetables come from the Brassica genus, including broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, kale, kohlrabi, mustard, rutabaga, turnips, bok choy, and Chinese cabbage (1). Arugula, horse radish, radish, wasabi, and watercress are also cruciferous vegetables.

Cruciferous vegetables are unique in that they are rich sources of glucosinolates, sulfur-containing compounds that impart a pungent aroma and spicy (some say bitter) taste (2). The hydrolysis (breakdown) of glucosinolates by a class of plant enzymes called myrosinase results in the formation of biologically active compounds, such as indoles and isothiocyanates (3). Myrosinase is physically separated from glucosinolates in intact plant cells. However, when cruciferous vegetables are chopped or chewed, myrosinase comes in contact with glucosinolates and catalyzes their hydrolysis. Scientists are currently interested in the potential for high intakes of cruciferous vegetables as well as several glucosinolate hydrolysis products to prevent cancer (see the articles on Indole-3-Carbinol and Isothiocyanates).

Disease Prevention

Cancer

Like most other vegetables, cruciferous vegetables are good sources of a variety of nutrients and phytochemicals that may work synergistically to help prevent cancer (4). One challenge in studying the relationships between cruciferous vegetable intake and cancer risk in humans is separating the benefits of diets that are generally rich in vegetables from those that are specifically rich in cruciferous vegetables (5). One characteristic that sets cruciferous vegetables apart from other vegetables is their high glucosinolate content (6). Glucosinolate hydrolysis products could help prevent cancer by enhancing the elimination of carcinogens before they can damage DNA, or by altering cell-signaling pathways in ways that help prevent normal cells from being transformed into cancerous cells (7). Some glucosinolate hydrolysis products may alter the metabolism or activity of hormones like estrogen in ways that inhibit the development of hormone-sensitive cancers (8).

An extensive review of epidemiological studies published prior to 1996 reported that the majority (67%) of 87 case-control studies found an inverse association between some type of cruciferous vegetable intake and cancer risk (9). At that time, the inverse association appeared to be most consistent for cancers of the lung and digestive tract. The results of retrospective case-control studies are more likely to be distorted by bias in the selection of participants (cases and controls) and dietary recall than prospective cohort studies, which collect dietary information from participants before they are diagnosed with cancer (10). In the past decade, results of prospective cohort studies and studies taking into account individual genetic variation suggest that the relationship between cruciferous vegetable intake and the risk of several types of cancer is more complex than previously thought.

Lung cancer

When evaluating the effect of cruciferous vegetable consumption on lung cancer risk, it is important to remember that the benefit of increasing cruciferous vegetable intake is likely to be small compared to the benefit of smoking cessation (11, 12). Although a number of case-control studies found that people diagnosed with lung cancer had significantly lower intakes of cruciferous vegetables than people in cancer-free control groups (9), the findings of more recent prospective cohort studies have been mixed. Prospective studies of Dutch men and women (13), US women (14), and Finnish men (15) found that higher intakes of cruciferous vegetables (more than three weekly servings) were associated with significant reductions in lung cancer risk, but prospective studies of US men (14) and European men and women (11) found no inverse association. The results of several studies suggest that genetic factors affecting the metabolism of glucosinolate hydrolysis products may influence the effects of cruciferous vegetable consumption on lung cancer risk (see Genetic influences) (16-21).

Colorectal cancer

A small clinical trial found that the consumption of 250 g/day (9 oz/day) of broccoli and 250 g/day of Brussels sprouts significantly increased the urinary excretion of a potential carcinogen found in well-done meat, suggesting that high cruciferous vegetable intakes might decrease colorectal cancer risk by enhancing the elimination of some dietary carcinogens (22). Although a number of case-control studies conducted prior to 1990 found that people diagnosed with colorectal cancer were more likely to have lower intakes of various cruciferous vegetables than people without colorectal cancer (23-26), most prospective cohort studies have not found significant inverse associations between cruciferous vegetable intake and the risk of developing colorectal cancer over time (27-32). One exception was a prospective study of Dutch adults, which found that men and women with the highest intakes of cruciferous vegetables (averaging 58 g/day) were significantly less likely to develop colon cancer than those with the lowest intakes (averaging 11 g/day) (33). Surprisingly, higher intakes of cruciferous vegetables were associated with increased risk of rectal cancer in women in that study. As in lung cancer, the relationship between cruciferous vegetable consumption and colorectal cancer risk may be complicated by genetic factors. The results of several recent epidemiological studies suggest that the protective effects of cruciferous vegetable consumption may be influenced by inherited differences in the capacity of individuals to metabolize and eliminate glucosinolate hydrolysis products (see Genetic influences) (34-37).

Breast cancer

The endogenous estrogen 17β-estradiol can be irreversibly metabolized to 16α-hydroxyestrone (16aOHE1) or 2-hydroxyestrone (2OHE1). In contrast to 2OHE1, 16aOHE1 is highly estrogenic and has been found to enhance the proliferation of estrogen-sensitive breast cancer cells in culture (38, 39). It has been hypothesized that shifting the metabolism of 17β-estradiol toward 2OHE1, and away from 16aOHE1, could decrease the risk of estrogen-sensitive cancers like breast cancer (40). In a small clinical trial, increasing cruciferous vegetable intake of healthy postmenopausal women for four weeks increased urinary 2OHE1:16aOHE1 ratios, suggesting that high intakes of cruciferous vegetables can shift estrogen metabolism. However, the relationship between urinary 2OHE1:16aOHE1 ratios and breast cancer risk is not clear. Several small case-control studies found that women with breast cancer had lower urinary ratios of 2OHE1:16aOHE1 (41-43), but larger case-control and prospective cohort studies did not find significant associations between urinary 2OHE1:16aOHE1 ratios and breast cancer risk (44-46). The results of epidemiological studies of cruciferous vegetable intake and breast cancer risk are also inconsistent. Several case-control studies in the US, Sweden, and China found that measures of cruciferous vegetable intake were significantly lower in women diagnosed with breast cancer than in women in the cancer-free control groups (47-49), but cruciferous vegetable intake was not associated with breast cancer risk in a pooled analysis of seven large prospective cohort studies (50). In a prospective study in 285,526 women, total vegetable consumption was not related to risk of breast cancer; individual subcategories of vegetable type, including cabbages, root vegetables, and leafy vegetables, were not individually associated with breast cancer in this cohort (51).

Prostate cancer

Although glucosinolate hydrolysis products have been found to inhibit growth and promote programmed death (apoptosis) of cultured prostate cancer cells (52, 53), the results of epidemiological studies of cruciferous vegetable intake and prostate cancer risk are inconsistent. Four out of eight case-control studies published since 1990 found that some measure of cruciferous vegetable intake was significantly lower in men diagnosed with prostate cancer than men in a cancer-free control group (54-57). Of the five prospective cohort studies that have examined associations between cruciferous vegetable intake and the risk of prostate cancer, none found statistically significant inverse associations overall (58-62). However, the prospective study that included the longest follow-up period and the most cases of prostate cancer found a significant inverse association between cruciferous vegetable intake and the risk of prostate cancer when the analysis was limited to men who had a prostate specific antigen (PSA) test (58). Since men who have PSA screening are more likely to be diagnosed with prostate cancer, limiting the analysis in this way is one way to reduce detection bias (63). Additionally, the most recent prospective study found that intake of cruciferous vegetables was inversely associated with metastatic prostate cancer—cancer that has spread beyond the prostate (i.e., late-stage prostate cancer) (62). Presently, epidemiological studies provide only modest support for the hypothesis that high intakes of cruciferous vegetables reduce prostate cancer risk (1).

Genetic influences

There is increasing evidence that genetic differences in humans may influence the effects of cruciferous vegetable intake on cancer risk (64). Isothiocyanates are glucosinolate hydrolysis products, which are thought to play a role in the cancer-preventive effects associated with cruciferous vegetable consumption. Glutathione S-transferases (GSTs) are a family of enzymes that metabolize a variety of compounds, including isothiocyanates, in a way that promotes their elimination from the body. Genetic variations (polymorphisms) that affect the activity of GST enzymes have been identified in humans. Null variants of the GSTM1 gene and GSTT1 gene contain large deletions, and individuals who inherit two copies of the GSTM1-null or GSTT1-null gene cannot produce the corresponding GST enzyme (65). Lower GST activity in such individuals could result in slower elimination and longer exposure to isothiocyanates after cruciferous vegetable consumption (66). In support of this idea, several epidemiological studies have found that inverse associations between isothiocyanate intake from cruciferous vegetables and risk of lung cancer (16-19) or colon cancer (34-36) were more pronounced in GSTM1-null and/or GSTT1-null individuals. These findings suggest that the protective effects of high intakes of cruciferous vegetables may be enhanced in individuals who more slowly eliminate potentially protective compounds like isothiocyanates. Alternatively, these same GSTs play a major role in detoxication of carcinogens and individuals with the null gene would be expected to be more susceptible to cancer; thus, the cruciferous vegetables may exhibit significant protection in this population if their protective effect is increasingly important at high carcinogen levels (67).

Nutrient Interactions

Iodine and thyroid function

Very high intakes of cruciferous vegetables, such as cabbage and turnips, have been found to cause hypothyroidism (insufficient thyroid hormone) in animals (68). There has been one case report of an 88-year-old woman developing severe hypothyroidism and coma following consumption of an estimated 1.0 to 1.5 kg/day of raw bok choy for several months (69). Two mechanisms have been identified to explain this effect. The hydrolysis of some glucosinolates found in cruciferous vegetables (e.g., progoitrin) may yield a compound known as goitrin, which has been found to interfere with thyroid hormone synthesis. The hydrolysis of another class of glucosinolates, known as indole glucosinolates, results in the release of thiocyanate ions, which can compete with iodine for uptake by the thyroid gland. Increased exposure to thiocyanate ions from cruciferous vegetable consumption or, more commonly, from cigarette smoking, does not appear to increase the risk of hypothyroidism unless accompanied by iodine deficiency. One study in humans found that the consumption of 150 g/day (5 oz/day) of cooked Brussels sprouts for four weeks had no adverse effects on thyroid function (70).

Intake Recommendations

Although many organizations, including the National Cancer Institute, recommend eating a variety of fruit and vegetables daily (serving number depends on age, sex, and activity level; see 71), separate recommendations for cruciferous vegetables have not been established. Much remains to be learned regarding cruciferous vegetable consumption and cancer prevention, but the results of some epidemiological studies suggest that adults should aim for at least five weekly servings of cruciferous vegetables (14, 58, 71).

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